Photochemistry of Surface Fresh Waters in the Framework of Climate Change

Abstract

Photochemical processes taking place in surface fresh waters play an important role in the transformation of biorecalcitrant pollutants and some natural compounds and in the inactivation of microorganisms. Such processes are divided into direct photolysis, where a molecule is transformed following sunlight absorption, and indirect photochemistry, where naturally occurring photosensitizers absorb sunlight and produce a range of transient species that can transform dissolved molecules (or inactivate microorganisms). Photochemistry is usually favored in thoroughly illuminated shallow waters, while the dissolved organic carbon (DOC) acts as a switch between different photochemical pathways (direct photolysis, and indirect photochemistry triggered by different transient species). Various phenomena connected with climate change (water browning, changing precipitations) may affect water DOC and water depth, with implications for the kinetics of photoreactions and the associated transformation pathways. The latter are important because they often produce peculiar intermediates, with particular health and environmental impacts. Further climate-induced effects with photochemical implications are shorter icecover seasons and enhanced duration of summer stratification in lakes, as well as changes in the flow velocity of rivers that affect the photodegradation time scale. This contribution aims at showing how the different climate-related phenomena can affect photoreactions and which approaches can be followed to quantitatively describe these variations.

A Critical View of the Application of the APEX Software (Aqueous Photochemistry of Environmentally-Occurring Xenobiotics) to Predict Photoreaction Kinetics in Surface Freshwaters

“…Therefore, the default calculation scenario concerns a well-mixed system of this kind. However, expert users could also employ APEX to successfully carry out calculations in stratified lakes and in rivers, also taking into account the effect of pH [27]. The tips and tricks to carry out these calculations are provided in dedicated sections (Sections 5 and 6).…”

“…However, lakes can become stratified during the summer season [79] while rivers flow by definition; thus, the most straightforward mathematical treatment of the reaction kinetics neglects some very important aquatic environments. Recently, tools and strategies have been developed to adapt the APEX output or input data to the treatment of somewhat more complex systems, such as stratified lakes, lakes undergoing evaporative concentration, and rivers [27]. The ways to do this, including tips on how to modify the code if needed, are reported below.…”

“…In the absence of water mixing, the epilimnion and hypolimnion evolve separately, and so do the photochemical reactions in the two compartments. Photoprocesses are faster in the epilimnion than in the average or mixed lake, while the reverse is true in the hypolimnion [27]. The sunlight irradiance and the sunlight spectrum incident on the epilimnion (i.e., the lake surface) are taken into account by the default p0sun column of the molecule.csv input file.…”

“…The water flow in rivers is definitely a major parameter as far as the importance of photochemical reactions is concerned [27]. The (average) flow velocity ν (units of [m s −1 ]) is a key issue, which is much more important for photochemistry when compared with the flow rate (Q, units of [m 3 s −1 ]), although the latter is more often available [81].…”

“…The main advantage is probably the software's flexibility, which has not only allowed for the modeling of pollutant phototransformation, but has also provided important insights into the photochemical functioning of natural surface waters [26]. Moreover, it has been possible to extend the model applications from the originally foreseen simple systems (stationary and well mixed ones, such as overturning lakes) to more complex environments such as stratified lakes, rivers, and to include additional phenomena such as evaporative water concentration [27]. This remarkable flexibility comes at a price, however, because ease of use has never been the strong point of APEX.Despite this drawback, research groups located at the Institut de Chimie de Clermont-Ferrand (France) [16] and the Säo Paulo State University (Brasil) [28][29][30] have used APEX with very remarkable results, but the software diffusion took place to a lesser extent than initially hoped for.…”